This invention relates to a method for making thermoplastic sticks or other shapes. The technique is particularly suitable for the manufacture of hot melt adhesive sticks, and, accordingly, it will be described in that context. It should be understood, however, that it can easily be employed to make other thermoplastic shapes.
The main advantage of hot melt adhesive is that it can be applied in liquid form, yet sets in seconds so that there is no appreciable slowdown in the production of the articles involved. Hot melt adhesive in stick form is preferred because the stick can be loaded conveniently into a gun-like applicator which melts the adhesive and dispenses it as a liquid bead onto the surfaces to be adhered. An applicator of this type is shown, for example, in copending application Ser. No. 509,335, filed Sept. 26, 1974, entitled ADHESIVE APPLICATOR TOOL, owned by the assignee of the present application. These adhesives and applicators are widely used, for example, in the shoe and packaging industries.
It has been found, however, that sometimes the sticks do not feed through the applicator properly unless they are of uniform consistency and diameter, e.g..+-. 0.010 inch. Improper feed causes the applicator to dispense a nonuniform bead resulting in adhesive skips or wastage. In some cases, the adhesive fouls the applicator so badly that the applicator breaks down. Thus, not only are maintenance costs increased, but an entire production line may have to be stopped until the problem is remedied or until a spare applicator can be brought on line.
It is difficult to make hot melt adhesive sticks of uniform size and shape because of the characteristics of the adhesives themselves. In liquid form, they are sticky. However, in air, the adhesive sets in a few seconds to a solid having a durometer on the order of fifty Shore, and while setting in a mold, an adhesive stick may shrink by as much as fifteen or twenty percent. Furthermore, the stick tends to set from the outside in, with the result that shrinkage also occurs along the axis of the stick. This, in turn, causes relatively large elongated wells or cavities to form in the ends of the stick, the sizes of these cavities increasing with increasing stick diameter. In addition, if the stick is released prematurely from a mold, it may settle under its own weight or bend. For these reasons, there are a large number of defective sticks. Further, since the material can only be recycled one or two times before it loses its correct adhesive characteristics, there is considerable wastage.
A conventional procedure for making shaped articles of thermoplastic material is shown, for example, in U.S. Pat. No. 3,723,035. Aside from encountering problems with the inherent stickiness of the material above its melting temperature and frequent tackiness below that temperature, prior techniques have not solved the shrinkage and cavitation problems discussed above and generally have not resulted in the efficient manufacture of uniform, solidified, hot melt adhesive sticks and shapes.
It has also been proposed to make adhesive sticks with continuous extruders of the type conventionally used to extrude thermoplastics. While the continuous extrusion process works fairly well with the higher melting point adhesives, difficulties are still encountered unless very precise cooling control is used. Moreover, such extruders are very expensive compared to batch-type equipment. Thus, instead of relying on more complex and expensive equipment, manufacturers have tended to minimize the effects of shrinkage and cavitation on the overall shape of the adhesive sticks by limiting the diameter of the stick. For example, one hot melt adhesive stick now in use is only one-half inch in diameter and about four to five inches long. This means that the person using an applicator accepting such sticks must stop quite often to reload. Not only is this troublesome to the operator, but it also slows down production.